Thin Film Bulk Acoustic Resonators (FBARs) are preferably fabricated to comprise layers having thicknesses that are within an acceptable range of so called "design" or nominal thicknesses. In this manner, when the FBARs are operated, they will exhibit series and parallel resonant frequencies (also collectively referred to as "resonant frequencies") which are within an acceptable error margin (e.g., within .+-.1%) of design or "target" series and parallel resonant frequencies, respectively. Due to the fact that thin-film layers which form FBARs are not always reproducible, however, these layers may not always be formed to have precise design thicknesses. As a result, for a case in which, by example, a plurality of FBARs are fabricated, some of these FBARs may exhibit resonant frequencies that are beyond an acceptable error margin of the target resonant frequencies.
High-quality production environments have been known to produce wafers having FBARs which exhibit series and parallel resonant frequencies ranging within .+-.1% of those yielded by other FBARs fabricated on a same wafer. However, FBARs fabricated on one wafer may not always exhibit resonant frequencies that are within this range of resonant frequencies yielded by FBARs formed on other wafers. By example, for a case in which there is a 3% variation in the thin film layer thicknesses of FBARs of a number of separate wafers, and wherein each FBAR comprises five layers, there can be a 7% variation in the resonant frequencies of these devices. As a consequence of this variation of resonant frequencies, a number of the wafers may have FBARs which yield resonant frequencies that are beyond the acceptable tolerance of the design resonant frequencies. By example, in a case wherein there is a standard deviation of 3% in the resonant frequencies yielded by FBARs from a number of different wafers, only 36% of the wafers may have FBARs yielding resonant frequencies ranging within .+-.1% of design resonant frequencies. Unfortunately, these disparities typically do not become apparent until after the FBARs have been separated from the wafer and their resonant frequencies have been measured.
In view of these problems, it can be appreciated that it is desirable to provide a technique which may be performed on FBARs after they have been formed on wafers, and which can be used to adjust or "tune" resonant frequencies exhibited by the FBARs in a manner that minimizes a disparity between the exhibited resonant frequencies and corresponding design resonant frequencies.
One known technique which attempts to tune resonant frequencies yielded by quartz crystal devices involves depositing a metal such as gold through a mechanical mask over selected portions of the devices. Unfortunately, this technique requires a photolithography step which would be difficult and expensive to perform on FBARs having bridge structures.
Thus, it is also desirable to provide a method for tuning resonant frequencies exhibited by FBARs that is inexpensive and simple to perform.